I had a friend who installed a Rostra cruise control on his 2006 ST1300. He found a left switch assembly for a Goldwing online with all the connections he needed for the Rostra cruise control unit. Rostra has been building automotive cruise control units for many years, so they are proven and reliable. The only thing is, they are built for cars. He mounted the Rostra into the left side of his fairing and ran the cable in front of the forks and under the right side of the tank. He fabricated a small bracket for the cable and another bracket to secure the end of the cable to the accelerator rod that adjusts the opening on all four carb's. He said the most difficult part was adjusting the cable for sensitivity. It took four tries (pulling the bike apart) to make each adjustment at the carburator shaft. He had to figure out the electronic connections between the Goldwing Switch assembly and the Rostra control unit, and run a bit of wire to get the length he needed to the right handlebar. Once he had it dialed in, it would allow the cruise to set the bike at speeds as low as just under 20 mph!! All settings (Set, Reset, Decel, Accel, disengage when brakes applied, etc) worked as though it was a factory installation. The use of the Goldwing switch assembly made the installation so "factory looking" that most people didn't even notice the switch assembly until it was brought to their attention. The other benefit to the use of the Goldwing Switch assembly is that it is sealed and virtually waterproof, unlike other aftermarket cruise switches at that time. He posted the whole story on an ST-Owners forum and it may still be accessible if anyone wants to find it.
Davo